Fiberboard (or particle board) products are formed by pressing and heating a mat of resin-coated wood fibers until the resin cures and adheres the wood fibers together, forming a solid wood-like product. Fiberboard is typically manufactured in the form of MDF (Medium Density Fiberboard). MDF has found its place in many industries as a low-cost, functional board alternative to more expensive solid wood. Although MDF may be used in many applications, its relatively low density of fibers (i.e., less than 0.7) is usually not sufficient to support high detail formed by emboss-pressing. To this end, a high-density fiberboard (HDF) must be used.
A conventional press used to compress a high-density fibrous mat and resin (or binder) to a particular molded shape includes two opposing platens which together define a molding cavity. Typically, at least one platen is heated through conduction, such as through the use of electric heating coils or by passing steam through appropriate conduits located in at least one of the platens. Although existing presses have been successful in making fiberboard products using only conduction heat (hot pressing), today's manufacturing demands require faster cycle times on the press and the use of stronger high-temperature resins to produce highly detailed, higher density, and, at times, thicker fiberboard products. A ten fold increase in the speed of curing of the furnish (or fibrous mat) may be realized by introducing steam into the mat. Steam may be introduced to the molding cavity from one platen. The injected steam passes through "channels" or interstitial spaces in the fibrous mat located within the molding cavity and is drawn from the mat by vacuum, or a suitable pressure differential, through appropriate openings and conduits provided in the opposing platen. This known cross-flow method of steam injection transfers the heat of the steam to the furnish forming the fibrous mat by heat-convection which effectively raises the core temperature of the mat quickly and uniformly and allows the resin to cure evenly and quickly.
One such steam-injection fiberboard production press is currently available from Sunds Defibrator, Inc. of Norcross, Ga. The press uses two steam distribution platens; an upper platen injects steam downward into the molding cavity and furnish while the opposing lower platen vents the steam (and any resulting condensation or moisture) from the bottom of the furnish.
Although this existing "cross flow" press design allows steam to heat all areas of the mat evenly and effectively, it precludes the use of an embossing platen wherein one surface of the cavity remains "clean", free of any injection nozzles, meshes, grooves, or openings so that high detail may be embossed on the surface of the pressed furnish.
U.S. Pat. No. 4,162,877, issued to D. W. Nyberg discloses a steam-injection fiberboard pressing system which includes two opposing press platens defining a molding cavity into which a fibrous mat is positioned and pressed to a desired shape. Only a lower platen is a fluid distribution platen which includes conduits and apertures to provide fluid communication between the molding cavity and both an external source of steam and a venting system, separated by controlling valves. The upper platen includes no injection or venting apertures or nozzles.
In operation of the system of U.S. Pat. No. 4,162,877, after a fibrous mat is positioned within the molding cavity, steam from the steam supply is introduced through the conduits and apertures of the lower platen and injected into the pressed fibrous mat located within the molding cavity. After a selected period of time, the control valves are operated to close off the supply of steam and thereafter to open the molding cavity to the venting system. The venting system uses the distribution platen to draw steam and moisture from the molding cavity.
Since the opposing (upper) platen of U.S. Pat. No. 4,162,877 is "clean", it may be used as an embossing platen to impress detail into the pressed fibrous mat, but only if the mat has a density less than 0.7. At any higher mat density, according to the patent, a mesh must be used to help prevent air from becoming trapped in the mat adjacent to the upper platen. Air trapped in the mat prevents proper curing of the resin binder and thus yields imperfections and weaknesses in the final product. Unfortunately, for many embossing-press applications, the density of the fibrous mat is greater than 0.7 and any use of a wire mesh, as taught by U.S. Pat. No. 4,162,877 would preclude the use of an embossing surface plate in the opposing platen.
Another problem with the press-platens disclosed in U.S. Pat. No. 4,162,877 is that they cannot allow for a continuous flow of steam from a remote source, through one platen, passing through a prescribed area of the mat located within the molding cavity, and leaving the mat through the same platen. Since both steam-injection and steam/moisture venting share the same system of conduits, passages and apertures located within the lower press platen of the system of U.S. Pat. No. 4,162,877, this press system must alternate between applying steam into the molding cavity through all the apertures and removing steam (and moisture) from the cavity through all the apertures.
It is an object of the invention to provide a fiberboard embossing press which overcomes the deficiencies of the prior art.
It is another object of the invention to provide such an embossing press which allows steam to flow continuously through a fibrous mat located within a molding cavity of the press and simultaneously to vent steam and moisture from the molding cavity while maintaining an embossing surface within the cavity.
In accordance with a preferred embodiment of the present invention, a distribution platen for use in a fiberboard press has a molding cavity and includes a steam injection conduit providing fluid communication into the molding cavity from a remote supply of steam. The distribution platen further includes a steam venting conduit which provides fluid communication from the molding cavity to a remote location. The press platen allows for the continuous flow of steam into the molding cavity through the dedicated steam injection conduit, and the continuous flow of steam and moisture from the molding cavity through the dedicated venting conduits.